Generally, in a construction machine such as an excavator comprising a revolving superstructure, a working device of a boom, an arm, a bucket, etc. that is attached to the revolving superstructure, and a hydraulic actuator including a revolving motor that drives the revolving superstructure and an actuator that drives the working device, a hydraulic oil tank made of iron, i.e., an iron hydraulic oil tank that is capable of storing therein pressure oil to be supplied to the hydraulic actuator described above is often provided in order to secure the strength of the tank. On the other hand, as in the case of a compact excavator, when installing a hydraulic tank together with other devices such as an engine, a hydraulic pump, a control valve, and a revolving motor on the revolving superstructure of which a space for installing devices is so small, a hydraulic oil tank made of synthetic resin is used because such a tank can be easily formed in the complex shape. A hydraulic oil tank made of synthetic resin requires less paints and anti-rust treatments and is lighter in weight than the hydraulic oil tank made of iron, and has the advantage that the heat of hydraulic oil stored therein is hardly transferred to an outside.
The patent document 1 discloses a structure for fixing such a hydraulic oil tank made of synthetic resin on a floor plate, that is, a hydraulic oil tank attachment structure. The conventional technique disclosed in the patent document 1 is configured such that a supporting pole passage for accommodating therein a supporting pole for fixing a hydraulic oil tank to a floor plate is provided in the inside of the hydraulic oil tank made of synthetic resin, of which side surfaces are formed in the shape of a plate.